Pelosi told her colleagues that if it appears likely that Bush wants to take the country to war against Iran, the House would take up a bill to deny him the authority to do so, according to Pelosi spokesman Brendan Daly.

That's from an article in Sunday's Washington Post. But actually, says Bruce Ackerman, a top constitutional scholar at Yale, it don't work that way:

BA: The president has to get another authorization for a war against Iran. It isn’t up to Nancy Pelosi or the House to prevent him; he doesn’t have the constitutional authority to just expand the war.

FP: What about actions short of invasion: air strikes or hot pursuit?

BA: Air strikes would be an invasion .... I think that the burden is very much on the president of the United States to ask for explicit authorization for an act of war against Iran. On every major military incursion, there is an elaborate ballet where the president says he has the power to do it and the Congress says, “You don’t have the power to do it.”

On a major incursion into another large Middle Eastern country ... the president will once again request the explicit authorization of Congress. When he was contemplating the invasion of Iraq, he was in a much stronger position politically—and he was still obliged to request authorization. And the same thing would happen again.

Ackerman may be optimistic here, given what we know about the Bush administration, and given Ayatollah Khamenei's repeated threat that Iran will retaliate if attacked. Matthew Yglesias outlines one scenario that could lead to war without Congressional authorization:

Say Israel bombs Natanz and Bush supports the Israelis diplomatically and warns Iran against retaliating. Then in Iraq there's some dramatic attack against US forces. In response, the President proclaims that the attacks were organized by Iran and orders, without first asking congress, a retaliatory bombing of Revolutionary Guard facilities. The military is going to obey that order, right? And congress isn't going to impeach and convict Bush, thus removing him from office, replacing him with Dick Cheney, right?